The present invention relates to a novel heterocyclic ring-containing compound, more precisely, a novel heterocyclic ring-containing compound utilized for a lubricant to be supplied between surfaces sliding with mechanical friction and so forth.
The performance required for a lubricant is that it can reduce friction coefficient of surfaces sliding with mechanical friction within a wide temperature range and wide pressure range, and such an effect should be maintained as long as possible. Further, a lubricant is required not only to have an effect of improving lubricity between members sliding with friction, but also to be able to impart wear resistance to the members sliding with friction themselves. The effect of lubricant such as engine oil for reducing frictional coefficient between members sliding with friction and a longer lifetime thereof directly lead to improvement in fuel consumption of mechanical driving, i.e., energy saving. Since a longer lifetime of engine oil makes possible not only reduction of the amount of waste oil but also reduction of exhausted CO2, it is preferred also in view of compatibility for environment, which is noted in recent years. Further, among sliding surfaces of industrial machines, if a conventional lubricant or grease is used for bearings or gears, which slide under especially severe frictional conditions, the lubricant may suffer from breakage of lubricant film or cause seizing when the lubricating conditions become severer, and thus a desired low friction coefficient may no longer be obtained due to wearing out damages. As a result, reliability of apparatuses may be degraded, and especially when a smaller apparatus is used, the friction conditions of sliding surfaces tend to become severer, which makes it difficult to use a smaller apparatus. Therefore, there is desired a energy-saving type lubricant that does not cause wearing out or seizing even under severe conditions and thereby enables improvement of reliability of apparatuses and contributes to use of a smaller apparatus.
Furthermore, in recent years, a lubricant supplied to surfaces of high density magnetic recording media, sliding surfaces or rotating members of micromachines and so forth is required that it can maintain such performances as mentioned above with an extremely small amount. That is, there is desired a lubricant that can cover a friction surface with a minimum amount to reduce friction coefficient of sliding surfaces and improve wear resistance, and can maintain such effects as long as possible. In order to respond to this requirement, a lubricant is inevitably required to have a property that it can readily form a uniform and smooth thin membrane.
Meanwhile, as lubricants, there have been conventionally used those containing lubricating base oil as a main component and blended lubricating aids such as organic compounds. As a typical example of the lubricating aids, diorganodithiocarbamic acid can be mentioned, and it is known that metal salts thereof show various functions as antioxidant, wear resistant agent, corrosion inhibition additive and so forth for lubricants. For example, the zinc salt disclosed in U.S. Pat. No. 4,278,587, antimony salt disclosed in U.S. Pat. No. 4,290,202, molybdenum salt disclosed in U.S. Pat. No. 4,360,438 and metals salts such as nickel, copper, cobalt, iron, cadmium and manganese salts disclosed in International Patent Publication in Japanese (Kohyo) No. 9-508156 have a remarkable effect that they can maintain low friction property and low wearing property of sliding surfaces even under severe conditions. In recent years, organic molybdenum compounds are particularly noted as lubricating aids. Organic molybdenum compounds are noted as materials that still show superior performances including wear resistance, extreme pressure resistance (withstand load property), low friction characteristics and so forth even when sliding members of mechanical apparatuses are moved under severe friction conditions caused by high temperature, high speed or low speed, high load, use of small size and so forth, and can effectively exert the lubricating performance under a pressure higher than ordinary pressure of fluid lubrication condition, i.e., a boundary lubrication condition.
However, it is known that the effect becomes more significant if zinc dithiophosphate is used together with the organic molybdenum compounds compared with a case of using the organic molybdenum compounds alone. Muraki, M., et al reported a mechanism that when a thin membrane of zinc dithiophosphate is formed on frictional surfaces, molybdenum dithiocarbamate or molybdenum dithiophosphate adsorbs on it, react with it and degrade to form a thin membrane of a mixture of molybdenum sulfide and molybdenum oxide (Tribologist, vol. 38, p. 10 (1993)). Arai, K., et al investigated elemental composition of surface portion of member sliding with friction along the depth direction by X-ray photoelectron spectroscopy (XPS) to confirm that molybdenum, sulfur and oxygen derived from molybdenum dithiocarbamate gradually decreased from the surface and iron elements increased conversely, and explained that low friction coefficient and wear resistance were obtained by formation of a composite film resulting from a reaction of metal iron on the sliding surface with molybdenum (Tribologist, vol. 44, p. 46 (1999)). Further, Kikuchi, T., et al. described that, besides zinc dithiophosphate, sulfur compounds such as sulfurized oil and fat, sulfurized olefin and sulfurized phenate also showed a synergistic effect with molybdenum dithiocarbamate for providing low friction property (JSAE Paper, 9537538 (1995)).
Although molybdenum dithiocarbamate is an outstanding material that exerts superior lubricating effect even under severe frictional conditions, a lubricant oil using it contains significant amounts of heavy metals such as molybdenum and zinc, sulfides that are easily oxidized to form sulfur oxide, which adversely affects not only the lubricating oil but also sliding members themselves as well as environment, and phosphoric acid, which eutrophicates rivers and the sea, and it is clearly unfavorable in view of compatibility for environment. Furthermore, the molybdenum oxide/molybdenum sulfide coating films formed on sliding surfaces are gradually shaved off by friction to newly form coating films, and therefore if either of the organic molybdenum compound and organic zinc compound that serve as sources of the coating films is depleted, the effect will be lost abruptly. On the other hand, if the amounts of the organic molybdenum compound and organic zinc compound are increased, by-products produced by shaving of the coating films increase in the system and adversely affect the apparatus itself comprising the sliding members. Therefore, increase of amounts of the compounds is not effective, and in fact, the effects such as improvement in fuel consumption brought by a longer lifetime of lubricant cannot be expected so much in a system utilizing the aforementioned organic molybdenum compound.
As described above, as conventional lubricants, there have not been provided yet any material that contains no environmentally harmful substances and environmental-polluting substances such as heavy metal elements, phosphate compounds and sulfides, exhibits superior performances as a lubricant, and can maintain the performances as a lubricant for a long period of time.
The present invention was accomplished in view of the aforementioned various problems, and its object is to provide a novel heterocyclic ring-containing compound, in particular, a novel heterocyclic ring-containing compound that shows superior lubricating performance not only in a state that it is mixed with conventional lubricant base oil, but also in a state that it is not mixed with lubricant base oil.
As an embodiment of the present invention, there is provided a heterocyclic ring-containing compound represented by the following formula (1). 
In the formula, D represents a heterocyclic ring residue having a 5- to 7-membered ring structure and substituted with (m+n) of substituents, X represents a divalent linking group consisting of a single bond, NR3 group (R3 represents a hydrogen atom or an alkyl group having 1-30 carbon atoms), an oxygen atom, a sulfur atom, a carbonyl, a sulfonyl or a combination thereof, R1 represents an alkyl group, an alkenyl group, an alkynyl group, an aryl group or a heterocyclic group, which may be substituted or unsubstituted, and R2represents a halogen atom, a hydroxy, an unsubstituted amino, a mercapto, a cyano, a sulfide, a carboxy or salt thereof, a sulfo or salt thereof, a hydroxyamino, a ureido or a urethane. m represents 1 or 2, and n represents an integer of 1 or larger. When m and n are 2 or larger, 2 or more of X, R1 and R2 may be identical to or different from each other or one another, respectively.
As another embodiments of the present invention, there are provided heterocyclic ring-containing compounds represented by the following formulas (2) to (8). 
In the formula, X1 and X2 each independently represent a divalent linking group consisting of a single bond, NR3 group (R3 represents a hydrogen atom or an alkyl group having 1-30 carbon atoms), an oxygen atom, a sulfur atom, a carbonyl, a sulfonyl or a combination thereof (preferably a sulfur atom or an imino (xe2x80x94NHxe2x80x94), more preferably imino). R11 and R12 each independently represent an alkyl group, an alkenyl group, an alkynyl group, an aryl group or a heterocyclic group, which may be substituted or unsubstituted, and R21 represents a halogen atom, a hydroxy, an unsubstituted amino, a mercapto, a cyano, a sulfide, a carboxy or salt thereof, a sulfo or salt thereof, a hydroxyamino, a ureido or a urethane (preferably a halogen atom). 
In the formula, R represents an alkyl group having 8 or more carbon atoms, an oligoalkyleneoxy group having 4 or more carbon atoms or a perfluoroalkyl group having 2 or more carbon atoms, R21 represents a halogen atom, a hydroxy, an unsubstituted amino, a mercapto, a cyano, a sulfide, a carboxy or salt thereof, a sulfo or salt thereof, a hydroxyamino, a ureido or a urethane, and p1 and p2 each independently represent 1 or 2. 
In the formula, R represents an alkyl group having 8 or more carbon atoms, an oligoalkyleneoxy group having 4 or more carbon atoms or a perfluoroalkyl group having 2 or more carbon atoms, and R21 represents a halogen atom, a hydroxy, an unsubstituted amino, a mercapto, a cyano, a sulfide, a carboxy or salt thereof, a sulfo or salt thereof, a hydroxyamino, a ureido or a urethane. 
In the formula, R and R21 are each same as those in the formula (3). 
In the formula, R, R21, p1 and p2 have the same meanings as defined in the aforementioned formula (3). 
In the formula, R and R21 have the same meanings as defined in the aforementioned formula (3). 
In the formula, R and R21 have the same meanings as defined in the aforementioned formula (3).
As embodiments of the present invention, there are provided the heterocyclic ring-containing compound wherein, in the formula (2), at least one of R11 and R12 contains xe2x80x94(Cxe2x95x90O)Oxe2x80x94; and the heterocyclic ring-containing compound represented by any one of formulas (3) to (8), wherein at least one of R contains xe2x80x94(Cxe2x95x90O)Oxe2x80x94.
According to the present invention, there can be provided novel and useful heterocyclic ring-containing compounds, in particular, novel heterocyclic ring-containing compounds that show superior lubricant performance not only in a state that they are mixed with conventional lubricant base oil, but also in a state that they are not mixed with lubricant base oil.
In another aspect of the present invention, this invention relates to use of a heterocyclic ring-containing compound represented by any one of the following formulas (1) to (8) for reducing friction coefficient between sliding surfaces.